http://www.ck12.org Chapter 16. Solutions
16.3 Colligative Properties
Lesson Objectives
- Define and give examples of colligative properties.
- Define vapor pressure and describe the conditions that affect it.
- Calculate changes in vapor pressure using Raoult’s law.
- Define boiling point elevation and be able to calculate the boiling point elevation for a particular solution.
- Define freezing point depression and be able to calculate the freezing point depression for a particular solution.
- Determine the molar mass of a solute from boiling point elevation or freezing point depression data.
Lesson Vocabulary
- vapor pressure: A measure of the force exerted by a gas above a liquid in a sealed container.
- colligative propery: Solution properties which are dependent strictly on the amount of solute added and not
the type of solute added. - Raoult’s law: States that vapor pressure of a solution equals the product of the vapor pressure of the pure
solvent and the mole fraction of solvent, as described by: P = xsolventP°. - mole fraction: The moles of a solution or mixture component divided by the total amount (in moles) of all
components in the solution or mixture. - boiling point elevation: A property describing the increase in boiling point observed when a solute is added
to a pure solvent. - freezing point depression: A property describing the decrease in freezing point observed when a solute is
added to a pure solvent.
Check Your Understanding
- Compare and contrast molarity and molality.
- How many moles of BaCl 2 are present in 250 mL of a 2.0 M BaCl 2 solution?
- Calculate the molality of a solution in which 50.0 grams of sodium nitrate have been dissolved in 1.00 L of
H 2 O.
Introduction
When a solute is dissolved in a liquid solvent to form a homogeneous solution, the behavior of the solution will often
be different from that of the pure solvent. For example, the boiling point, freezing point, and vapor pressure of the
solution will be different from that of the original solvent. The freezing point of salt water is lower than the freezing
point of pure water, and the boiling point of a sucrose solution is higher than that of pure water. In this lesson, we
will look at how to calculate the magnitude of these changes based on the concentration of the solution.